Methylsulfoxide radicals CH3SOx (x = 1-4) are key reactive sulfur species (RSS) in the atmospheric oxidation of volatile organic sulfur compounds (VOSCs). Through flash vacuum pyrolysis (FVP) of trifluoromethanesulfinic acid methyl ester CF3S(O)OCH3 at 1000 K, the methoxysulfinyl radical CH3OSO has been generated in the gas phase and subsequently characterized in cryogenic N-2, Ar, and Ne matrices by IR spectroscopy. Upon 266 nm laser irradiation, CH3OSO efficiently isomerizes to the less stable methylsulfonyl radical CH3SO2 in matrices without noticeable decomposition. In the gas phase, CH3OSO reacts with O-2 and yields sulfinyl peroxyl radical CH3OS(O)OO, a new member in the CH3SOx, (x = 1-4) family. This radical dissociates into SO3 and CH3O with the intermediacy of the sulfonyoxyl radical CH3OSO3 under the 266 nm laser irradiation. Additionally, the photoisomerization of CF3S(O)OCH3 to sulfenic ester CF3OSOCH3 was also observed.